Project Summary Late-life depression (LLD) is a common mental disorder in the elderly, with prevalence rates ranging from 1 to 5%. Recent evidence suggests that LLD is linked to age-related negative health outcomes, such as cerebrovascular disease, increased risk of Alzheimer's disease, vascular dementia, and of premature mortality. The mechanisms of LLD are complex and involve the dysregulation of different biological pathways. Understanding the interplay between the biological changes in aging and depression can provide insight into the mechanisms by which LLD increases the risk of negative health outcomes. This study proposes to evaluate the association of Senescence-Associated Secretory Phenotype (SASP) Index with different clinical phenotypes of aging (i.e., cognitive impairment) and with cellular senescence phenotype (i.e., leukocyte telomere [LT] attrition) in LLD. Finally, we will evaluate the trajectory of changes in SASP, and its relationship with cognitive performance in these individuals. Our hypotheses are that LLD individuals will show a significantly higher SASP index compared to age- and gender-matched never-depressed control subjects. SASP index will be significantly associated with greater cognitive impairment and telomere attrition in LLD subjects. We further hypothesize that an increasing or persistently higher SASP index trajectory will lead to faster cognitive decline among study participants over two years of follow-up. To our knowledge, this will be the first study to examine the association between circulating molecular senescence markers (SASP), a cellular senescence marker (LT attrition), and neurocognitive and clinical characteristics in LLD. Based on the results of this study, we will also be able to identify novel targets for the development of interventions aiming not only the treatment of depression in the elderly but also aiming the prevention of the negative outcomes related to this condition.